The invention relates to an apparatus for pneumatically feeding at least one spinning preparation machine, for example a carding machine or cleaner. Pneumatic feed means are known in which feeding is accomplished by means of fibre-flock charging shafts, which are located upstream of the individual spinning preparation machines and which are connected to a pneumatic fibre-transporting line, the transporting line being connected by way of a fibre material feed fan to the fibre-processing machine located upstream, and wherein automatic modification of the amount of air is arranged to be carried out.
In practice, radial fans are used for feeding material to charging shafts through a pneumatic transporting line, the air-related technical properties of those fans being described by a set of characteristic curves. The characteristic curves describe the volumetric air flow in dependence upon the total pressure difference being produced at the fan. A multiplicity of curves for various speeds of rotation of a fan is accordingly produced. If the total pressure difference being produced is then altered by external factors such as different charging pressures at material shafts, the volumetric air flow will consequently also vary in accordance with the characteristic curve. However, varying volumetric air flows have an extremely disadvantageous effect on the ability to adjust the material feed. Because relatively high volumetric flows are necessary on the intake side and low volumetric flows are desired on the feed side, a volumetric flow that is as constant as possible is desirable. However, because of the different charging pressures at a material shaft, the volumetric flow is not constant. Customarily, the volumetric air flow is determined by means of differential pressure measurement at a volumetric flow measurement nozzle and is displayed. The fan speed of rotation is set to a fixed value by means of a frequency converter. This is usually carried out only when the system is being re-set and involves a considerable amount of work. Nevertheless, even when a system is in a preselected, desirable state of operation, variations in the charging conditions still occur, caused by changes during operation. The pressures in the individual shafts, which vary because of changing charging conditions in the fibre material shaft, especially due to the material requirements of the downstream processing machine (e.g. carding machine), give rise to varying volumetric air flows because of variations in the pressure in the transporting line.
In a known apparatus (EP 303 023), provision is made for modified control of the amount of flocks fed into a conveying system when the number of fibre-processing carding machines changes. When such a change is made, for example when a number of carding machines are switched off, it is not necessary to modify the amount of air by appropriately displacing the characteristic curve of the fan; the amount of conveying air adjusts itself automatically to the number of carding machines in production. Accordingly, the possible operating points according to the set of characteristic curves for the fan are not held in the control system. It is consequently not possible for the actual value of the amount of air to be compared to the desired value for the operating point. In particular, the operating point of the fan is dependent upon the total pressure difference, that is to say it is also dependent upon the pressure on the intake side of the fan and, in the known apparatus, that pressure is not measured. Only the static pressure at the fan outlet is measured, which pressure is used for regulating the amount of fibre flocks in the transporting line. It is disadvantageous therein that—in the case of a constant number of carding machines—it is not possible to carry out automatic modification in accordance with the pressure variations in the fibre material shafts and consequently it is not possible to make the amount of conveying air more uniform.
In contrast thereto, it is an aim of the invention to provide an apparatus of the kind mentioned at the beginning which avoids or mitigates the mentioned disadvantages and which especially can keep the volumetric flow for the conveying of fibre material substantially or completely constant in spite of different charging pressures at at least one fibre material shaft.